1. Field of the Invention
The invention relates to a fuel injection system for use in internal combustion engines.
2. Description of the Prior Art
For the sake of better comprehension of the description and claims, several terms will first be explained: The fuel injection system of the invention is embodied as pressure-controlled. Within the scope of the invention, a pressure-controlled fuel injection system is understood to mean that by means of the fuel pressure prevailing in the nozzle chamber of an injection nozzle, a nozzle needle is moved counter to the action of a closing force (spring), so that the injection opening is uncovered for an injection of the fuel out of the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called the injection pressure, while the term system pressure is understood to mean the pressure at which fuel is available or is kept on hand inside the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. The term leakage is understood to be a quantity of fuel that occurs in operation of the fuel injection (such as a reference leakage or diversion quantity) that is not used for the injection and is returned to the fuel tank. The pressure level of this leakage can have a standing pressure, and the fuel is then depressurized to the pressure level of the fuel tank.
In common rail systems, the injection pressure can be adapted to both load and rpm. To reduce noise, a preinjection is often performed. To reduce emissions, a pressure-controlled injection is known to be favorable.
In pressure-controlled systems, a triangular injection course results in the main injection. The nozzle needle closes in response to the drop in pressure in the nozzle chamber. It has been demonstrated that a fast closure (rapid spill) of the nozzle needle is advantageous. This rapid closure can be attained in pressure-controlled fuel injection systems by means of a fast relief of the nozzle chamber. However, the pressure reduction should not proceed so fast that the injection pressure is already reduced while the nozzle needle is still open because of its inertia. That would cause a blowback of combustion gases into the nozzle chamber. By the reinforcement of the needle closure, the relief of the nozzle chamber can proceed more slowly, so that cavitation damage caused by overly rapid relief of the nozzle chamber is avoided.
The hydraulic reinforcement of the closing performance causes a fast pressure reduction in the nozzle chamber and thus faster closure of the nozzle needle. The closure, hydraulically reinforced according to the invention, of the pressure-controlled nozzle needle can also be employed for fuel injection systems with a pressure booster, for the sake of improved pressure reduction and refilling. It is advantageous to place the relief valve as close as possible to the nozzle chamber. Another advantage in terms of the closing performance is attained by having the diversion valve communicate not directly with the leakage line but rather via the spring chamber of the injection nozzle. To optimize the relief performance, a throttle can additionally be disposed at the outlet of the nozzle chamber. One additional valve for performing the hydraulically reinforced closure of the nozzle needle can be dispensed with, if for that purpose the diversion flow from the metering valve is used for the fuel injection.